Lead aluminate pigment and paint and method of making same



Patented Mar. 24,1942- LEAD ALUMINATE PIGMENT AND mm'r AND METHOD orMAKING SAME Louis 1:. Barton, Windsor, Conn.

No Drawing. Application March as, 1939,

, Serial No. 264,583

(or. 23-52), I,

which this application is a continuation-in-part, I I'havedescribed aprocess of making hydrated.

a 20 Claims.

My invention relates generally tam-pigments, I

their preparation and application, and more specifically to a new andimproved type of white pigments-the aluminates of lead-together withimproved methods of making same and their application in paints.

In my co-pending application for patent Serial No. 41,867, filed in theU. S, Patent Office September 24, 1934, I have described an improvedmethod of making white aluminatesof certain metallic elements, which maybe summarized as consisting in heating an intimately blended mixturecontaining an oxide of the metallic element and the oxide of aluminum toa temperature sufficiently high to form the aluminate, but withoutfusion of the charge. i-

The white aluminate pigments so prepared are mostly stable to the actionof water and other in-- fiuences, and also havephysical properties whichmake them suitable for use in paints and for other applications wherewhite pigments are required. However I have foundthat in the case oflead aluminate pigments, the degree of stability to the action of waterdepends upon the quality of materials used, audit is one of the objectsof my invention to provide an improved method of making lead aluminateswhereby the product is stabilized so'as to impart the desiredproperties.

In; my ,co-pending application for patent Serial Number 270,013, filedApril 25, 1939,.the'preparalead aluminate pigments of improved stabilitythat will not discolor when treated ,with water, which, brieflydescribed, consists in -heating an intimate mixture of lead oxide andaluminum oxide to formation of lead aluminate inthe solid phase withoutfusion and then thoroughly washing the calcined product and drying it.It was stated in the application referred to that in addition to thelead aluminate pigments of regular molecular formula, there shown indetail, others of any desired composition could be prepared. It was alsostated that lead aluminate pigments can be prepared at temperatureslower than the safe temperature range" specified. It is the object ofthis application not only to show the working of my process in preparingstable lead aluminates of regular molecular formula but also to show howthe stabilizing process applies to lead aluminates of other compositionsand to lead aluminates made at the lower temperatures referred to as.well as those made in the stated tion of lead aluminate pigments ,isparticularly described and discussed. My process of making leadaluminate pigments there described, in brief,

consists in making a niixture of such lead and aluminum compoundsas'will, when heated, supply to the product onlyslead and aluminumoxides and then heating the mixture in the temperature range about 725'to 880 C. until the oxides of lead andaluminum' chemically combine byreaction in the solid phase. Such general process may optionally bemodified by precalcining the alumina, as'there described, before mixingit with the lead compound'to yield, after calcining'the mixture, a leadaluminate of improved or perfect stability.

" I have furthermore discovered that an imj proved lead aluminatepigment can be made by controlled hydration of the product alreadystabilized by precalcining the alumina as before described. Such processand product I have fully disclosed and claimed in my (lo-pendingapplicaticn for patent Serial No. 270,012; filed in the United StatesPatent Office April 25, 1939.

In my co-pending United States patent application Serial No. 129,817,filed March 9, 1937, of

safe temperature range.

sults in a hydration of the pigment when subjected to the action ofwater, the actionbeing ac celerated at elevatedtemperatures. Thehydration, if not excessive, is not detrimental and to some extent isadvantageous as will be shown later, but when the hydration is excessiveit is accompanied by the development of -a brown or gray color whichbecomes progressively darker with longer continued action of water.

With themore unstable lead aluminates the degree of instability may beroughly determined by observing the time required for noticeable colorchanges. \However, the degree of instability is best judged bydetermining the amount of hydration that results from subjecting thepigment to the action of water for a given time. I have furthermorefound that lead aluminate pigments prepared as described in my.co-pending patent application Serial No. 41,867 referred to, may,

after long storage, acquire a brownish color indicative of theinstability of the pigment under I atmospheric conditions.

If anhydrous alumina (AlzOsLhaving the socalled alpha structure, is usedin making the lead aluminate, the product will be perfectly stable; butin that case, if the aluminum oxide has been calcined at too high atemperature, the reaction between the lead and aluminum oxides takes.

place very slowly, so as to make the process more expensive if notimpracticable. Such product is The instability of lead aluminatepigments realso liable to be of poor color or relatively less white.

If aluminum hydrate, or aluminum oxide, which has been prepared bycalcining aluminum hydrate at temperatures below 1000 C. is used inmaking the lead aluminate, the reaction between the lead and aluminumoxides will proceed rapidly to completion so as to yield a whiteproduct, but one which is relatively less stable. Such 'a product is infact sufiiciently stable to be wetmilled and dried as described in mypending ap-.

plication Serial Number 41,867, but the production of a perfectly stablelead aluminate pigment has distinct advantages over the less stableproduct. I

- As a product, stable lead aluminate has the advantage in variousapplications in the arts, many of which require water or aqueoussolutions, or involve the use of water in preparation of the product,that there is no danger of hydration or possible discoloration. In theprocess of manufacturing lead aluminates the stable products have thedistinct advantage in that they may be handled in continuous systems ofwet-milling and classification, which may subject the pigment to theaction of water for many days. It is permissible also to use highertemperatures in drying the wet-milled product.

In making lead aluminate, as described in my pending application SerialNo. 41,867, the upper limit of temperature range is only a little above850 C. hence if the so-called gamma aluminum dicated the stabilizingeffect of longer calcining of the aluminate and washing the product asby wet-milling. The washing incident to wet-milling, described in myco-pending patent application referred to, may, ormay not, besufficient, depending, apparently, upon quality of materials used, theirtreatment in process and amount of impurity to be removed.

I have now discovered that if the washing be sufficiently thorough, ahydrated lead aluminate pigment containing a material content ofcombined water and having the desired stability can be made either whenusing aluminum hydrate in the process or alumina which has not beenprecalcined at a sufficiently high temperature to assure stability. Suchthoroughly washed pigment is not only stabilized against discolorationin the presence of water but is stabilized against discoloration undernormal atmospheric conditions during storage.

It is well known that commercial aluminum hydrates and oxides containsmall amounts of alkali and other compounds and that lead compounds mayalso contain impurities. The stabilizing of the lead aluminate bywashing the cal cined pigment is believed to be largely attributable tothe complete or substantially complete elimination of water-solublecompounds by washing the calcined pigment, though it may be in partattributable to physical changes in the product resulting in a morestable form.

The thoroughly washed pigment is not necessarily absolutely free fromalkali since it appears that a part is sometimes in fixed cheniicalcombination insoluble in water; also the practical impossibility ofremoving every trace of soluble material from a finely divided solid iswell recognized. What I mean by thorough washing, however accomplished,is substantially complete elimination of water-soluble compounds, suchtreatment resulting in a hydrated lead aluminate pigment, containing amaterial content of combined water and thereafter not subject toexcessive hydration and not discolored by the action of water nor whenkept under normal atmospheric conditions in storage.

In carrying out my improved process, I prefer to wet-mill the calcinedpigment for about one hour, or sufficiently long to break down allaggregates and assure maximum contact between liquids and solids.Instead of simply dewatering the slurry from the wet-mill and drying, asis usual, I prefer either to wash the product by settling anddecantation with several successive portions of water or by a continuouswashing system. If washed by settling and decantation, usually four suchwashings are sufficient, after which the solids are separated, as byfiltration or other means, and finally dried. If a continuous system ofwashing is employed, such 'as a series of Dorr tanks, the amount ofwater required to wash to the same extent may be larger butsuch practicepermits classification of product if desired.

It is obvious that other equivalent means of I washing the calcinedpigment may be used as by successive wet-milling operation; or bywet-milling followed by filtration and washing in .the filter press; orby wet-milling followed by successive centrifugings. It is also possibleto wash finely divided calcined products without wetmilling.

I have also found that separately wet-milling the aluminum and leadcompounds before blending them, or thorough washing of the blendedmixture before calcining, improves the stability of the pigment to someextent, but I prefer to wash the calcined product as described, suchprocedure being both more effective in stabilizing the pigment andeconomical in operation.

ing subjected to the The products of my improved process, after beactionof water, are hydrated to an extent depending upon the time and tosome'extent upon the nature of materials used. For a given stock ofmaterials the time factor can be determined and the hydration controlledby regulating the time the products are subjected to the action of waterto yield products within a preferred range of 0.25 to 2.00% of combinedwater, or, the products may be hydrated to a greater extent if desired.

I have furthermore discovered that lead aluminate pigments, whether ornot perfectly stabilized against discoloration by the action of water,if hydrated asindicated are, as compared with anhydrous products, ofbetter hiding power and greatly improved in dispersion properties andwetability with vehicles, such as linseed oil. When mixed and ground,the pigment and vehicle readily combine to form a smooth paste, which,upon further dilution with the vehicle, forms a paint of creamyconsistency that works easily under the brush and spreads to an evenfilm in which the dispersion of pigment particles is remarkably uniform.

I have furthermore discovered that a stable anhydrous leadaluminatepigment can be made r by re-calcining the hydrated pigment for a upminutes at temperatures preferablyabout 400 C.

By my improved methods I can make normal and basic lead aluminate,PbAl2O4 and PbcAlzOs, or lead aluminates of intermediate basicity suchas PbSAhOD. It is evident that I may prepare by my improved methodsaluminates of any de-.

sired composition or containing-an excess of. either alumina or thebasic element; the various aluminates, basic sub-basic, normal andsubnormal, adjusting during the heating process proportionally in theproduct in accordance with natural laws to the state of most stableequilibrium. In case'either the aluminum or lead oxide is in largeexcess, the product may contain some of the excess oxide in solidsolution or in.

the free state or both. Bythe expression essentially pure in the claims,I refer to lead aluminates consisting pre nderatingly of chemicallycombined lead and aluminum oxides, and containing no oxides other thanlead and aluminum except it be traces of impurity containedin the rawmaterials used and combined water.

Before giving specific examples of my inven tion and discoveries, I willfirst give 'ageneral description of my improved methods covering certainfeatures which are common in the preparation of all my novel leadaluminate pigments. As a source of aluminum oxide, Imay use alumina(Ala-0a), or any aluminum compound which upon heating yields aluminumoxide, such as aluminum hydrate or aluminum nitrate, but

I prefer to use aluminum hydrate for, economicreasons. i Asa source oflead oxide, I may use thc'oxide as such, or any compound of lead thatwill yield in the preparation 01' lead aluminate, I may use eitherlitharge (PbO), or redlead (Pba04), or

lead peroxide (PbO-z), or white lead (2 Pb CO:.

PbHaOa). J

I then thorou hly mix the alumina with the lead compoundin the requiredproportions either by dry milling, or, preferably, by wet-milling,

'thereby reducing particle size and also assuring later in the examplesof operation.

White lead aluminate pigments can be made in the temperature range 725C. to 8'70 0.

though prolonged heating is required at 725 C.. and the highertemperature, 870 C., is, so near to fusion point that very closetemperature control is required. A temperature'range of 750 to 850 C. issatisfactory but for greatest economy I prefer a temperature range of800 to 850 C. The progress of the reaction during calcining may bereadily judged by withdrawing. a small 7 sample from the furnace forobservation. The lead aluminates will-be colored when the reaction isincomplete, but will be substantially white in either hot or coldcondition when the reaction is complete.

Some of the lead aluminatesare sufllclently fine, as discharged from thecalcining furnace.

if care has been used not to exceed the" upper limit ofthe calciningtemperature range, as to enable them to beused as pigments for paints,

'- but usually milling is required to break down aggregates and also toreduce the particlesize.

Wet-milling methods with or without classification are particularlysuitable for this process the products of which are stabilized bywashing as I have already described.

the required .oxide'upon heating. For example,

a very close association of the ingredients.. The

milling may'be done either in a batch or a continuous type of mill;in'either case -I prefer "to use a porcelain or silex lining to avoidcontamination of materials. I

The slurry from the wet-mill is dewateredasby settling and decantationand/or'by filtration. Any suitable equipment may be used, suchasDontanks and filter presses.

The wet pulp is then charged directly into the heating furnace, or itmay be preliminarllydriedand then charged into the furnace. The furnacemay be either of the batch type, such as a mullle,

hearth furnace or Bruckner furnace, or of the continuous kiln type. Ineither case a lining of high-grade flre brick is satisfactory to avoidcontamination of materials. Whatever the type of fumace used, meansshould be provided for close control of temperature as the upper limitof temperature range for lead aluminates is but-little.

below the fusion point of lead oxide and lead aluminate. In furnaceswhere products of combustion contact the charge a neutral or oxidizing-flame should be maintained to avoid possible reduction of metallicoxides.

The time required in the calcining zone de-- pends upon the temperature.The higher the temperature within the permissible range, the shorter thetime. However, there is tendency to clearer color tones at the lower andmedium tem- .tion of my improved method.

- Th following examples are given as illustrative of the operativeprocedures I use and of the characteristics of some of the products, butshould not be regarded .as limiting the broad "EXAMPLE L-Normal leadilluminate.

'A mixture of 121 parts 51 aluminum hydrate.

equivalent to '18 parts of aluminum oxide, and

171 parts of leadfoxide',-'PbO, was prepared ,by wet-milling thematerials: together forone hour and thendew'atering and drying themixture.

' The mixture was then calcined for 2 hours inv I the temperature range,830 to 850 C. c The .calcined productha'd a white color. A

small sample treatedwith cold water acquired 1 a strong gray color after48 hours treatment.

The remainder of the calcined product was; wet-milled for one hour andthen washed four times with'cold water by settlingand decantation washedslurry was then'dewatere'd and the pigwithina period of- 10 hours. Thethoroughly Portions of the finished pigment treated with "cold water forperiods of 10 and 30 days retained their white color and after treatmentshowed combined water, hydration, of 2.15 and, 6.14%

respectively.

- EXAMPLE '2.Normal lead aluminat e A mixture of 121 parts of aluminumhydrate,

equivalent to 78 partsof aluminum oxide, and 171 parts of lead oxide,PbO, was prepared by erat'um within the permissible range. mas;

. required at various temperatureswill be indicated applica jvwet-milling the materials together for one hour and then dewatering anddrying the mixture.

The mixture wasthen calcined for 2 hours in r the temperature range,'830to 850 C.

The calcined product had a white color. A small sample treated with coldwater acquired a strong gray color after 48 hours treatment.

The remainder of the calcined product'was wet-milled for one hour andthen washed four times with cold water by settling anddecantati'onwithin a period of hours. The thoroughly washed slurry was thendewatered and the pigment dried at 90 to 100 C.

Chemical composition Per cent Lead oxide 67. 80 Aluminum oxide 31. 12Combined water (hydration) 1. 08

Physical properties Specific gravity a. 5. 90 Color White Oilabsorption-parts oil per 100 parts ofpigment 16 Tinting strength-whitelead standard:

Portions of the finished pigment treated with cold water for periods of10 and 39 days retained their white color and after treatment showedcombined water, hydration, of 2.16 and 6.20% respectively.

EXAMPLE 3.-Normal lead aluminate A mixture of 482 parts of aluminumhydrate,- equivalent to 31.4 parts of aluminum oxide, and 68.6 parts oflead oxide, PbO, was prepared by wet-milling the materials together forone hour andthen dewatering and drying the mixture.

The mixture was then calcined for 2 hoursat a temperature of 850 C. to870 C.

A small portion of the calcined product was treated with cold water:

. cold .water and After 48 hours. treatment the product had a gray colorand contained 3.95% of combined water. 1

. After 9 days treatment theproduct had a strong gray color andcontained 12.86% of combined water.

The main portion of the calcined product was wet-milled for 1 /2 hoursand then washed seven times with water by settling and decantationwithin a period of 8 hours. The washed pigment was dried at atemperature of 90 to 100 C. I

' Chemical composition of finished pigment Per cent Lead oxide 67. 52Aluminum oxide 30.84 Combined-water 1.64

' Portions of .thefin'ished' pigment treated with dried had thefollowing proper- Exmru: 4.-+Norfnal lead aluminate- A mixture of 48.2parts of aluminum hydrate, equivalent to 31.4 parts of. aluminum oxide,and 68.6 parts of lead oxide, PbO, was prepared by 2,277,061 wet-millingthe materials together for one hour and then dewatering and drying themixture.

The mixture was then calcined for 12 hours at a temperature of 730 C. to750 C.

' A portion of the white calcined product was treated with cold water.After seven days the gray product after dryingcontained 2.92% combinedwater.

The main portion of the'calcined product was wet-milled for one hour andwashed four times by settling and decantation within a period of 10hours and then dried at 90 to 100 C.

The finished pigment contained 1.40% of combined water, or hydrationduring wet-milling, washing and drying.

A portion of the finished pigment was treated with cold water. After 20days treatment the .pigment had not discolored and after dryingcontained 3.66% of combined water.

ExAMP'LE 5.N0rmal lead aluminate A mixture of 78 parts of aluminumoxide, I which had been pre-calcined at 1050 C., andl'll parts of leadoxide, PbO, was prepared by wetmilling the materials together for onehour and then dewatering and drying the mixture.

The mixture was then calcined for 2 hours in the temperature range 830to 850 C.

The calcined product had a'white color. A small sample treated with coldwater acquired a slight gray color after 10 days treatment. Thehydration during the 10 days treatment was The remainder of the calcinedproduct .was wet-milled for one hour and then washed four times withcold water by settling and decantation within a period of 8 hours.

Portions oi? the finished pigment treatedwith cold water for periods of10 and 30 days retained their white color and after treatment showedcombined water, hydration, of 0.98 and 4.12 respectively.

EXAMPLE. 6.Basic lead aluminate A mixture of 57 'parts, of aluminumhydrate,

equivalent to 37 .parts of aluminum oxide, and

163 parts 01' lead oxide, PbO. was prepared by wet-milling the materialstogether for one ,hour

ties: Percent After-l8 hours, no discoloration, combined water(hydration) 2.61 After- 9 days, no discoloration, combined water(hydration) 3. 32 After 20 days, no discoloration, combined water(hydration), 5.

and then dewatering and drying the mixture.

The mixture was then calcined for 2 the temperature range 810 to 830 C.

The calcined product had a cream white color. A small sample treatedwith cold water acquired a strong gray color after treatment for 6 days.

Theremainder of the calcined product was wethours in milled tor one'hourand then washed four times with cold water by settling and decantationwithin a period'of 10 hours. The'thorouglily washed slurry was thendewatered and the pigment dried at to C.

The thoroughly Portions of the finished pigment treated with cold waterfor periods'ot and days retained their white color and after treatmentshowed combined water, hydration, of 1.40 and 3.58 re- 7 spectively.

Exmras 7. Subnormal lead aluminate A mixture of 58 parts of white leadcarbonate,

equivalent to 50 parts of lead oxide. and 77 parts of aluminum hydrate.equ valent to 50 parts of aluminum oxide, was prepared by wet-millingthe materials together for one hour and then dewatering and drying themixture. K

The mixture was calcined for 26 hours'at a temperature of 725 C.

The calcined product was wet-milled for one hour and washed four timesby settling and decantation within a period of 8 hours and thendewatered and dried at 90 100 C Chemical composition of finished pigmentPer-cent Lead oxide 49.45 Aluminum oxide 49.45 Combined water 1.10

Physical properties Specific gravityn gn 5.26 V Color White Tintingstrength-white lead standard: a 100 -60 A portion of the finishedpigment was treated with cold water for seven days without discolora-'tion. Aiter drying,.the treated material contained 3.36% combined water.

Exmra8-Sub-basic lead aluminate A mixture of '15 parts of lead oxide,PhD, and 35.5 parts of aluminum hydrate, equivalent to 25 parts-ofaluminum oxide, was prepared by wetmilling the materials together forone hour and then dewatering and drying the mixture:

The mixture was then calcined for 2 hours at a temperature of 820 C. to840. C.

A small portion of the calcined product showed a gray discolorationafter treatment with cold water for '72 hours. After treatment for 7days the product, after drying, contained 2.90% of combined water.

The main portion of thecalcined wet-milled 'for one hour and washed fourtimes by settling and decantation within a period of 10 product was2,277,061 5 Chemical Philsical properties Lead oxide l 80.72 Specificgravity.-.'--;. '.i.. 6.90 Aluminum oxide- ;-a.. 18.40' Color J WhiteCombined water (hydration) 0.88 5 strengthwhite-lead-standard= y noPhsicdl 0 m V J 1 v Apcrtion oi the flnishedpigment wastreated Specificgrant with cold water for seven dayswithout discolora- Cream white tion.After drying, the treated material conou zg agw or on per 9 i 14 i0tained 1.22% combined water;

As shown in the foregoing. examples, the-hydration is not necessarilyproportional to the time treated with water but apparently depends alsoonthe quality; andconditionof materials and their treatment in process;therefore for any stock of materials and combination of process factorsthe. time treated with water would be varied as determined by trial togive the degree of hydration desired. However the results in theforegoing examples do'show: v

(1) That the time employed in wet-milling and washing can beregulated toyield a pigment containing combined water (hydration) in amount withinthe preferred range of 0.25 to 2.00% and that the time available forsuch milling, washing and drying is ample for industrial operation ofthe Process. a (2) When the calcined lead aluminate is promptly andthoroughly washed, the hydration of the pigment is thereafter at aslower rate thus permitting control of combined water content of theproduct. This is clearly indicated in Exam:

ples 3, 4, 5 and 8. Thus in Example 3, when thecalcined product wassimply treated with cold water it hydrated to 3.95% combined water in 48hours and to 12.86% combined water in 9 days. After wet-milling,thorough washing and drying, the pigment contained 1.64% of combinedwater.

Portions of the finished pigment similarly treated with cold water forperiods of 48 hours and 9 days contained respectively 2.61% and 3.32%combined water; therefore showing an additional hydration during thoseperiods of 0.97 and 1.68%

- respectively. Even after 20 days treatment with cold water thecombined water was only 5.80%, showing an additional hydration of only4.16%. (3) By thorough, washing, the lead aluminate pigment isstabilized against discoloration, showhours and then dewatered and driedat to b c. v i J Chemical composition of finished pigment Per cent LeadflYidP 74.56 Aluminum oxide g 24.86 Combined waters .58

ing no change in color regardless of the time of treatment with water orthe extent of hydration.

My improved lead pigments are insoluble in} and substantially unaffectedby water, and though as a group, they are attacked'by concentratedacids, and alsoto some extent by dilute reagents depending upon variety,they are relatively resistant to chemical action as would be expectedfrom chemical combinations formed at high temperatures. It may thereforebe inferred that they are relatively stable and less toxic than whitelead carbonate,

Two classes of white pigments are generally recognized: (I) The opaquewhite pigments, such as white lead, zinc oxide and titanium pigmentswhich impart opacity and whiteness when mixed with oil or other organicvehicle; and (2) the extender pigments such as whiting and china .50%basic lead aluminate clay which do not impart appreciable opacity whensimilarly mixed with vehicles.

The opaque pigments of class 1 are further subdivided into two classes:Those which are reactive or capable of combining chemically orphysico-chemically with ingredients of the vehicle, such as white leadand zinc oxide; and (b) those which are chemically inert, such astitanium pigments.

It is well known that the reactive pigments have a special value, notpossessed by inert pigments, of enhancing certain desirable propertiesin paints and other products to which they differ in degree ofreactivity. This is quite apparent from the characteristics of paintfilms'to be discussed later, but it is definitely indicated in the caseof the lead aluminates by. their capacity to accelerate'the drying ofraw linseed oil when mixed with it and spread as paint in a mannersimilar to the drying action of white lead. This is' shown roughly, in aquantitative way by the following test and results.

Mixtures of various lead aluminates and-alsoof' white lead carbonatewere made with raw linseed oil using the same proportion of pigmentpigments are used, such as for rubber, linoleum and other vfloorcoverings, printing inks, vitrified enamels, etc. 7

In the field or paints for exterior use much investigationhas beenconducted for many years in an eilort to combine the best features ofthe various opaque white pigments, but while much progresshas been made,the improvement in one i'eatur'e has too often been accompanied by aloss of other desirable qualities.

Heretofore white lead carbonate was the only white opaque pigment which,without admixture of other pigments would make a reasonably satisfactoryexterior paint with linseed oil vehicle. Zinc oxide desirably hardensthe paint film, but when used without admixture of other pigments willcheck and crack: admixture with white lead is the usual practice.Lithopone pigment because chemically unstable to atmospheric influencesfails rapidly and is little used in high grade exterior paints. Titaniumpigments are in great demand for their excellent hiding power, but failrapidly .by chalking when used alone in exterior paints.

The manufacturers of paints now recommend :admixture of titaniumpigments with white lead,

zinc oxide or both, thereby improving the paint to acertain extent.However, titanium pigments with white lead and linseed oil vehicle,while durable, rapidly accumulate dirt and mildew. The addition of zincoxide prevents dirt collection and mildew, but in a short time leads tochalking and consequent fading of tinted paints.

to vehicle by volume. The mixtures were spread,

p y as rather thick coatings, on' a steel plate as nearly as possible atthe same spreading rate. The time for the films to become dry to thetouch was recorded in hours as follows:

Basiccarbonate white lead 52 Normal lead aluminate 110' Basic leadaluminate (PbaAhOa) 47 Basic lead aluminate (Pb2Al2O5) 86 I (PbzAlzOs)50% titanium barium pigment 48 From the foregoing results, it should benoted Hours that the normal lead aluminate, though accelerating thedrying of linseed oil more than is the case with an inert pigment, isrelatively feebly reactive, while the basic lead aluminates are morereactive than white lead. The result showing the accelerating action ofbasic lead aluminatewhen mixed with a titanium pigment is particularlyimportant from a practical application standpoint as well as stronglyindicating:

the reactive properties of lead aluminate.

In the enumerated examples before given, no reference has been made tocolor, since while all are white in the sense of their capacity to.

produce a white paint, the undertone depends much upon the calcin'ationconditions'as before explained. At lower calcination temperature normallead aluminate has a clear white with a faint cream undertone. The creamundertone of the basic lead aluminate is slightly stronger than in thenormal product.

From the foregoing properties of lead aluminate pigments, they are as agroup adapted for application to oil, enamel and lacquer type paints,and also for other purposes where white Chalking and fading of tintedpaints also occurs within a, short time when mixtures of titaniumpigments and zinc oxide are used with a linseed oil vehicle.

From what I have heretofore stated about lead aluminate pigments, it isevident that hiding power, being in most cases in the order of whitelead in that respect, is not of special importance,

though the hiding power is sufiicient to makepaints which will cover orhide'the surface with the usual number of coats. The outstandingcharacteristic of lead aluminate pigments is their capacity to impartdurability to paints.

I have discovered that paints made with my lead aluminate pigmentsground with raw linseed oil vehicle are exceptionally durable whenexposed to light and weather, retain gloss longer, and do not chalk fora longer period than is the case with white lead.

I have also found that exterior paints made with mixtures of leadaluminate and titanium pigments in a raw linseed oil vehicle areremarkably durable when exposed to light and weatherbeing highlyresistant to chalking, fading of tints, checking and cracking; they alsoretain their gloss very well and are free from dirt collection andmildew so as to retain clean surfaces For the first time to my knowledgethis result has been accomplished with a'titanium containing paint in araw linseed oil vehicle.

The cleanness of surface and freedom from dirt collection and mildew ofpaints containing lead aluminates, particularly the basic leadaluminate, I attribute to the fact that the paint films dry to a hardersurface as compared with similar paint combinations made with white leadwhich dry to a film which remains slightly tacky to the touch for a longtime after application.

I have furthermore discovered that a lead aluminate mixed withzinc-oxide in a linseed oil paint prevents the checking and crackingwhich occurs where zinc oxide only is used as the pigwhite, stabilized,hydrated lead aluminate pig- I pigment which consists in making amixture of ment in exterior paint. I have also found that basic leadaluminate mixed with lithopone in a linseed oil vehicle greatly improvesthe durability of exterior lithopone containing paint.

For exterior paints made with mixed pigments and raw linseed oilvehicle, I prefer to have the lead aluminate about 50% of the pigmentcom bination, but lesser amounts may be used with good results andimprovement roughly proportional to the particular percentage used. V I0I have also found a marked improvement in paints where lead aluminate isadded to a plurality of pigments, either a mixture of opaque 1 pigments,or a mixture containing a colored or tinting pigment, or an e tenderpigment which is often added to increa e the pigment volumeconcentration or to cheapen the paint.

I have furthermore discovered that lead aluminate pigments giveremarkable protection to iron and steel under exposure to light, weather2 and other influences. This result I attribute to the reactivenature ofthe pigments, apparently functioning like free oxides of lead, such asred lead which for many years has been used with linseed oil as paintwhere maximum protection of iron or steel is desired. Thus, for thefirst time, a white pigment suitable for use with linseed oil as aprotective coating for iron and steel is available. j

. I-claim as my invention:

l. The method of making an essentially pure, white, stabilized, hydratedlead aluminate pigment which consists in making a mixture of such 7 leadand aluminum compounds as will, upon; heating, supply to the productonly lead and 3:,

aluminum oxides, heating the mixture until the said oxides chemicallycombine in the solid phase without fusion and then thoroughly washingthe product with 'water to substantially eliminate water-solublecompounds to form said stabilized .m hydrated lead aluminate pigment.

2. The method of making an essentially pure,

ment containing to 81% lead oxide, computed 1 on the basis of theanhydrous product, the balance being aluminum oxide, which consists inheating a mixture of lead oxide and aluminum hydrate 'inthe temperaturerange 725 C. to i 870 C. until the oxides of lead and aluminumchemically combine in the solid phase, thoroughly 50 washing the productwith water to substantially;-

eliminate water-soluble compounds to form' said stabilized hydrated leadaluminate pigment.

3. The method of making an essentially pure,

white, stabilized, hydrated leadaluminate p18- ment, which consists inmaking a mixture of such lead and aluminum compounds as will, uponheating, supply to the product only lead and aluminum oxides, heatingthe mixture in the temperature range 725' C. to 870 C. until the saidoxides chemically combine in the solid phase without fusion and thenthoroughly washing the product with water to substantially eliminatewater-soluble compounds to form said stabilized, hydrated, leadaluminate pigment.

4. Themethod of making an'essentially pure, white, stabilized, hydratedbasic lead aluminate 1 725 to 870 C. untilthe said oxides chemicallycombine in the solid phase without fusion and 78 to substantiallyeliminate water-soluble coinpounds to form said stabilized hydratedbasic lead aluminate.

5. The method of'making an essentially pure,

' white, stabilized, hydrated subnormal lead aluminate pigment whichconsists in making a mixture of such leadand aluminum compounds as will,upon heating, supply to the product only PhD and A: with less than onemolecular equivalent of PhD to each molecular equivalent of A1203,heating the mixture in the temperature range 725 to 870 C. until thesaid oxides chemically'combine in the solidphase without fusionand thenthoroughly washing'the product with water to substantially eliminatewater-soluble compounds to form said stabilized hydrated subnormal leadaluminate.

6. An essentially pure, white, hydrated, colorstable leadaluminate-pigment having .a material content of combined water andcharacterized by its substantial freedom from water-soluble compoundsand permanent absence of discoloration either when treated with water orwhen exposed to the atmosphere.

7. Anessentially pure, white, hydrated,

2.00% of combined water and characterized by its substantial freedomfrom water-soluble compounds and permanent absence of discolorationeither when treated. with water or when exposed to the atmosphere.

a. An essentially pure, white, hydrated, tomstable basic lead aluminatepigment having a material content of combined water and char-' acterlzedby its substantial freedom from watersoluble compounds and permanentabsence of discoloration either when treated with water or whenexposedto the atmosphere.

9. An essentially pure, white. drated, colorstable subnormal leadaluminate pigment havor when exposed to the atmosphere.

10. An essentially pure, white, hydrated, colorstablelead aluminatepigmentcontaining 0.25 to' 2.00% of combined water and characterized'by.

its substantial freedom from water -soluble compounds and permanentabsence of discoloration either when treated with water or when exposedto the atmosphere, such pigment containing,

when computed on the basis of-the anhydrous product, 50 to 81% of leadoxide (PbO) and the balance aluminum oxide;

11. The method of making an essentially pure,

white hydrated lead aluminate pigment'having improved hiding poweranddispersibility in vehicles which consists in making a'mixture of suchlead and aluminum compounds, as will, upon heating, supplyto the productonly lead and aluminum oxides, heating the mixture until the said oxideschemically combine in the solid phase without fusion. and then treatingI the product with water for a time sufficient to form said hy- .dratedlead aluminate pigment but insuflicient I to discolor theproduct.

12. The method of making an' essentially pure, white, lrvdrated normallead aluminate pigment I having improved hiding power anddispersibilityin vehicles which consists in making a mixture of such lead and aluminumcompounds as will, upon heating, supply to the product only lead andaluminum oxides in proportion to form colorstable lead aluminate pigmentcontaining 0.25 to p A1203 with more than one molecular equivalent ofPhD for each molecular equivalent of A1203,

heating the mixture in the temperature range 725 C. to 870 C. until thesaid oxides chemically combine in the solid phase and treating theproduct with water for a time suilicient to form said hydrated basiclead aluminate pigment but insufiicient to discolor the product.

14. The method of. making an essentially pure,

white, hydrated subnormal lead aluminate pigment having improved hidingpower and dispersibility in vehicles which consists in making a mixtureof such lead and aluminum compounds as will, upon heating, supply to theproduct only PhD and A120: with less than one molecular equivalent ofPhD for each molecular equivalent of A1203, heating the mixture in thetemperature range 725. C. to 870 C. until the said oxides chemicallycombine in the solid phase and treating the product with water for atime sufllcient to form said hydrated subnormal lead aluminate pigmentbut insufilcient to discolor the product.

15. The method of making an essentially pure, white, hydrated leadaluminate pigment having improved hiding power and dispersibility invehicles and containing 50 to 81% of lead oxide (PbO), computed on thebasis of the anhydrous product, the balance being aluminum. oxide, whichconsists in making a mixture of lead oxide minate pigment butinsuflicient to discolor the product. a

16. The method of making an essentially pure, white, hydrated leadaluminate pigment having improved hiding power and dispersibility invehicles, which consists in making a mixture of suchlead and aluminumcompounds as will, upon heating, supply to the product only lead andaluminum oxides, heating the mixture in the temperature range 725 C. to870 C. until the said oxides chemically combine in the solid phasewithout fusion-and treating the product with water for a time suflicientto form said hydrated lead aluminate pigment but insuflicient todiscolor the product.

17. A substantially pure, white, opaque lead aluminate pigment made bythe process of claim 11, said pigment consisting essentially ofchemically combined lead and aluminum oxides and a material content ofcombined water.

18.A substantially pure, white, opaque lead aluminate pigment made bythe process of claim 11, said pigment consisting essentiallyrofchemically combinedlead and aluminum oxides and 0.25 to 2.00% ofcombined'water.

19. A substantially pure, white, opaque basic lead aluminate pigmentmade by the process of claim 11, said pigment consisting essentially ofchemically combined lead and aluminum oxides and a material content ofcombined water;

20. A substantially pure, white, opaque subnormal lead aluminate pigmentmade by the process of claim 11, said pigment consisting essentially ofchemically combined lead and aluminum bxides anda material contentofcombined water.

" LOUIS E. BARTON.

